The precipitation of inorganic salts, such as alkaline earth metal sulfates, from produced brines to form scale is a persistent and common problem encountered during the production of hydrocarbons from subterranean reservoirs. Scale deposition in the reservoir, in the hydrocarbon production well bore penetrating the reservoir, and in surface and subsurface production equipment and tubing is typically caused by commingling incompatible fluids in situ and producing the resulting brine during hydrocarbon recovery operations, especially during enhanced oil recovery (EOR) operations involving waterflooding or a water drive.
Aqueous fluids are deemed incompatible if each fluid contains distinct ions which ultimately form a precipitate that deposits as a scale when the fluids are commingled. A common example of two incompatible fluids is a connate water containing naturally-occurring alkaline earth metal cations and an EOR injection water containing naturally-occurring sulfate anions. Offshore operations often involve the injection of large volumes of sea water containing relatively high concentrations of sulfate anions into subterranean reservoirs containing connate water with relatively high concentrations of alkaline earth metal cations. Upon in situ mixing of the sea water and connate water, precipitation of alkaline earth metal sulfate scale often occurs in the reservoir, in the well bore, or in surface and subsurface production tubing and equipment.
To remedy the problem of scale deposition, conventional commercial scale inhibitors, primarily comprising polyelectrolytes such as polycarboxylates or polyphosphonates, are added to the brine in situ. The scale inhibitors advantageously maintain the inorganic salt in solution and prevent precipitation of the scale-forming salt from the produced brine during hydrocarbon production operations. Nevertheless, it is often desirable to precipitate and separate the inorganic salts from the produced brine after production of the hydrocarbons, particularly where the inorganic salt is radioactive, to reduce the disposal load. Unfortunately, the downstream presence of a scale inhibitor in the produced brine maintains the inorganic salt in solution and hinders separation of the inorganic salt from the brine.
Accordingly, it is an object of the present invention to provide a process for removing an inorganic salt from an aqueous brine. It is further an object of the present invention to provide a process for removing an inorganic salt from an aqueous brine in the presence of a scale inhibitor. It is a yet another object of the present invention to provide a process for removing an inorganic salt from an aqueous brine produced from a subterranean hydrocarbon-bearing reservoir, wherein the salt is a radioactive alkaline earth metal salt.